DefenseMechanisms

Defense Mechanisms

The Importance of Defense Mechanisms

• Corals are sessile
  • Fixed at a certain position, attached to a substrate (such as a rock, or between sand)
  • Corals are sessile, colonial animals — remaining stationary for most of their life cycle — which makes the ocean a very dangerous place
  • Their vulnerability has lead to some of the most lethal toxins found in nature today
    • chemical defense is vital

• Toxicity was naturally selected for
  • Corals that were often preyed upon by fish now have higher toxicity levels, for their own protection
  • The low nutritional value of some corals made them less susceptible to predation, so the lower the nutritional value, the lower the toxicity level

Chemical Defense Mechanisms

• Toxins
  • Harmless to humans (except fire coral – can cause pain, inflammatory effects)
  • Most toxins are neurotoxins
    • Interfere with signal transmission in animals’ nervous systems
    • Three main types
      • Saxitoxin – causes paralysis and respiratory failure
      • Palytoxin - causes kidney, respiratory and heart failure
      • Lophototoxin – causes muscle contractions, possibly paralysis and respiratory failure

• Symbiotic Relationships
  • A close relationship between two species
  • Corals often live in symbiosis with bacteria and protists that produce toxins, using them for protection
    • Coral probiotic hypothesis
  • Some corals maintain symbiotic relationships with small animals
    • Trapeziid crabs and stony coral

• Nematocytes
  • Stinging cells used to capture small prey, kill of neighboring corals in a continuous battle for space
  • Most corals possess these in addition to everything else

Physical Defense Mechanisms

• Cnidocils
  • Activated when a predator touches it
  • Discharges a nematocyst
• Nematocysts
  • Discharge by firing a barb into the predator, leaving a hollow filament through which poisons are injected to immobilize the prey
  • Tentacles move the prey to the polyp mouth